The present invention relates to semiconductor light emitting devices made of Group III–V nitride semiconductors, which are capable of emitting light in the blue to ultraviolet regions.
Recently, light emitting diodes (GaN-based LEDs), using a Group III–V nitride (hereinafter, referred to simply as a “nitride”) expressed by a general formula Bz ,AlxGal−x−y−zInyN, where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, have found wide application in various kinds of display panels, large display apparatus and traffic lights, for example. White LEDs, in which a GaN-based LED is combined with a fluorescent substance, have also been put into practical use, and are expected to replace the currently used lighting equipment, if their luminous efficiency is improved in the future.
FIG. 18 illustrates a cross-sectional structure of a known blue light emitting diode in which nitride semiconductors are used (See Japanese Laid-Open Publication Nos. 07-094782, 10-173224, and 2000-5891.) As shown in FIG. 18, in the known blue light emitting diode, a first semiconductor layer 102 made of an n-type nitride semiconductor, and a second semiconductor layer 103 made of a p-type nitride semiconductor are sequentially stacked on a substrate 101 made of sapphire.
A first electrode 104 made of nickel and gold with a thickness of from about 2 nm to about 5 nm is formed on the second semiconductor layer 103. The first electrode 104 can make a good ohmic contact with the p-type nitride semiconductor.
A second electrode 105 made of gold is formed on the first electrode 104. The second electrode 105, which is for wire bonding, passes through the first electrode 104 to reach the second semiconductor layer 103. An n-type ohmic electrode 106 is formed on an exposed portion of the first semiconductor layer 102.
With this structure, in the known blue light emitting diode, recombination radiation (generated light), emitted by the pn junction formed by the interface between the first and second semiconductor layers 102 and 103, is transmitted through the second semiconductor layer 103 and the first electrode 104, and then extracted.
However, a problem with the known blue light emitting diode is that the recombination radiation produced by the pn junction is partially absorbed by the first electrode 104 made of the metals. To deal with this problem, if the thickness of the first electrode 104 is reduced significantly, the amount of radiation transmitting through the first electrode 104 can be increased. In that case, however, a trade-off occurs in which series resistance (sheet resistance) in the first electrode 104 is increased, which makes it difficult to significantly increase the optical electric characteristics of the device, that is, the device characteristics.
Alternatively, instead of the metals, transparent material may be used to form the first electrode 104 in order to increase the light-extraction efficiency. Nevertheless, a problem also arises in this case, in which contact resistance between the p-type nitride semiconductor layer and the transparent electrode formed thereon is large.
In addition, there is another problem in that nitride semiconductors, in which the activation ratio of an impurity, particularly of a p-type impurity, that determines the conductivity type of the semiconductor is small, have large sheet resistance.